In small electronic appliances such as note-type personal computers, smartphones, mobile phones, etc., which have been provided with increasingly higher performance and more functions, electronic devices such as microprocessors, imaging chips, memories, etc. should be mounted densely. Accordingly, to prevent malfunction due to heat generated by them, the dissipation of heat generated from such electronic devices has become increasingly important.
As a heat-dissipating sheet for electronic devices, JP 2006-306068 A discloses a heat-conductive sheet comprising at least a graphite film and an adhesive resin composition, which is a reaction-curable vinyl polymer. The graphite film is (a) expanded graphite formed by an expanding method, or (b) obtained by heat-treating a polyimide film, etc., at a temperature of 2400° C. or higher. The expanded graphite film is obtained by immersing graphite in acid such as sulfuric acid, etc. to form a graphite interlayer compound, heat-treating the graphite interlayer compound to foam it, thereby separating graphite layers, washing the resultant graphite powder to remove acid, and rolling the resultant thin-film graphite powder. However, the expanded graphite film has insufficient strength. Also, the graphite film obtained by the heat treatment of a polyimide film, etc. is disadvantageously expensive despite high heat dissipation.
JP 2012-211259 A discloses a heat-conductive sheet comprising graphite pieces, which comprise pluralities of first graphite pieces obtained by thinly cutting a thermally decomposed graphite sheet, and second graphite pieces smaller than the widths of the first graphite pieces, at least the first graphite pieces connecting both surfaces of the heat-conductive sheet. This heat-conductive sheet is obtained, for example, by blending the first and second graphite pieces with a mixture of an acrylic polymer and a solvent, and extruding the resultant blend. However, the extruded heat-conductive sheet does not have sufficient heat dissipation, because of a high volume fraction of the resin.
JP 2006-86271 A discloses a heat-dissipating sheet as thick as 50-150 μm comprising graphite bonded by an organic binder having a glass transition temperature of −50° C. to +50° C., such as an amorphous copolyester, a mass ratio of graphite/organic binder being 66.7/33.3 to 95/5. This heat-dissipating sheet is produced by applying a slurry of graphite and an organic binder in an organic solvent to a parting-agent-coated film on the side of a parting layer, drying the slurry by hot air to remove the organic solvent, and then pressing it, for example, at 30 kg/cm2. JP 2006-86271 A describes that the pressing of a graphite/organic binder sheet improves its thermal conductivity. In JP 2006-86271 A, a slurry of graphite and an organic binder in an organic solvent is applied by one operation. It has been found, however, that the application of an entire slurry by one operation provides non-uniform distribution of graphite. In addition, because a mass ratio of graphite to an organic binder is not so high in Examples (80/20 in Example 1, and 89/11 in Example 2), sufficiently high thermal conductivity inherent in graphite is not exhibited.
JP 11-1621 A discloses a high-thermal-conductivity, solid composite material for a heat dissipater comprising highly oriented graphite flakes and a binder polymer polymerized under pressure. This solid composite material is produced by mixing graphite flakes with a thermosetting monomer such as an epoxy resin to prepare a composition comprising at least 40% by volume of graphite, and polymerizing the monomer while compressing the composition under sufficient pressure to align graphite substantially in parallel. JP 11-1621 A describes that a volume fraction of graphite in the composite material is preferably 55-85%, though it may be from 40% to 95%. However, the distribution of graphite flakes is non-uniform in an epoxy resin containing graphite flakes in a high concentration of 95%. Thus, JP 11-1621 A describes only experimental results at a graphite flake volume fraction of 60%.
JP 2012-136575 A discloses a conductive, heat-dissipating sheet comprising organic particles made of polyamides, acrylic resins, etc. and having an average particle size of about 0.1-100 μm, conductive inorganic fillers having an average particle size of about 10 nm to about 10 μm, and a cured resin such as an epoxy resin, etc., organic particles/inorganic fillers being 1000/1 to 10/1, and the percentage of inorganic fillers being 5-30% by weight based on the total amount. JP 2012-136575 A illustrates graphite, coke, carbon black, etc. as inorganic fillers, though only carbon black is used in Examples. However, because the percentage of conductive carbon black is as small as 5-30% by weight, the conductive heat-dissipating sheet of JP 2012-136575 A does not have sufficient heat dissipation.
As described above, conventional heat-dissipating sheets containing a small percentage of graphite or carbon black do not have sufficient heat dissipation despite uniform distribution of graphite or carbon black. Increase in the percentage of graphite or carbon black results in lower sheet strength despite improved thermal conductivity, particularly causing the problem of easy detachment of graphite or carbon black from the heat-dissipating sheet.
In addition, it has been found that a high percentage of graphite particularly causes non-uniform distribution. Because heat-dissipating sheets produced industrially are usually cut to predetermined shapes and sizes and then disposed in small electronic appliances, the non-uniform distribution of graphite provides cut heat-dissipating sheets with unevenness in performance.
Accordingly, inexpensive heat-dissipating sheets having uniform, high heat dissipation as well as mechanical properties necessary for handling are desired.